Genetic variation in mahoganies - Springer Link

Biodiversitv and Conservation 2, 114-126 (1993)

Genetic variation in mahoganies: its importance, capture and utilization A D R I A N C. NEWTON* and R O G E R R.B. L E A K E Y Institute of Terrestrial Ecology (ITE)~:, Bush Estate, Penicuik. Midlothian, Scotland EH26 OQB, UK

J. F R A N C I S C O MESt~N Proyecto Meloramiento GenOtico, Centro Agronomico Tropical de Investigacion y Ensegtanza (CA 77E), Turrialba, Costa Rica

Received 11 June 1992; accepted 30 July 1992 Mahoganies (Swietenia spp. ; Meliaceae) are amongst the most economically important tropical hardwoods, and yet little information exists concerning their patterns of genetic variation. The characterization of this variation is essential for defining more accurately the conservation status of mahogany populations, and for their economic utilization. The loss of genetic variation through deforestation may be critical for these species, which are highly susceptible to pest attacks when grown in plantations. This paper assesses the current state of knowledge concerning the extent of genetic variation in mahoganies, and highlights its potential importance. It is suggested that any conservation strategy developed for mahoganies should include a genetic selection and improvement programme as well as the protection of natural stands in situ. Techniques by which particular genotypes may be captured for ex situ conservation are briefly described. Keywords: mahogany; genetic conservation; propagation

Introduction

Although mahoganies (Swietenia spp. ; Meliaceae) are amongst the most commercially important hardwoods in the neotropics (Lamb, 1966), little attention has been paid to the extent of genetic variation that exists within the natural distribution of these species. Characterizing this variation is of importance for defining both the in situ and ex situ conservation status of particular populations, and for the development of afforestation and tree improvement programmes. The conservation status of mahogany species in the neotropics has recently been the subject of increasing concern, as natural populations are currently being severely depleted by deforestation (Read, 1990). As an illustration of this, the United States and Costa Rican governments recently proposed the genus Swietenia for listing in Appendix II of the Convention on International Trade in Endangered Species (CITES) (US CITES proposal, 1992), The aim of this review is to survey the known extent of genetic variation that exists in neotropical mahoganies, particularly with respect to the results of genetic tests which have been performed to date. The importance of this variation for the conservation and

*To whom correspondence should be addressed. *A component of the Edinburgh Centre for Tropical Forests. 0960-3115 © 1993Chapman & Hall

Genetic variation in mahoganies

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economic utilization of these species is discussed, and the techniques that enable certain genotypes to be conserved ex situ are briefly described. Although concerned primarily with the neotropical mahoganies in the strict sense (Swietenia spp.), a number of points will be illustrated by reference to closely related species within the Meliaceae, from both the Americas and other geographical regions. The extent of genetic variation in wild populations

Taxonomic definition of species The taxonomy of Swietenia has been described in detail in two recent monographs (Pennington and Styles, 1975; B.T. Styles in Pennington, 1981). The genus is represented in the neotropics by three species, S. macrophylla King, S. humilis Zuccarini and S. mahagoni (Linnaeus) Jacquin. These species are interfertile, which hinders the attribution of many individuals to a particular species in some areas of their natural distribution. For example, naturally occurring hybrids of S. humilis x S. macrophylla occur in N.-W. Costa Rica (Holdridge and Poveda, 1975; Whitmore, 1983); variation in characteristics such as leaf morphology and fruit size is continuous in this area (Eugenio Corea, personal communication). Similarly, the hybrid between S. macrophylla and S. mahagoni arose spontaneously after the introduction of S. macrophylla to a number of Caribbean islands (Whitmore and Hinojosa, 1977; B.T. Styles in Pennington, 1981). The putative hybrid S. humilis x S. mahagoni has also been recorded (Whitmore and Honojosa, 1977). The ability to hybridise greatly influences the genetic composition of individual populations. For example, the Swietenia population present in Puerto Rico effectively constitutes a hybrid swarm, with hybrids and backcrosses between the two main species (see Whitmore and Hinojosa, 1977). The occurrence of such hybrids has important implications for genetic conservation strategies, as well as for tree improvement research. For example, the introgressive hybridization caused by the presence of S. macrophylla on Caribbean islands such as Puerto Rico represents a form of genetic erosion ofS. mahagoni. It is conceivable that pure genetic stocks of both species could eventually disappear in such localities. In terms of intraspecific genetic variation, chromosome races have been identified in Swietenia as well as other species of Meliaceae (Styles and Khosla, 1976). Both diploid (2n = 54) and tetraploid (2n = 108) races occur in S. macrophylla, and a polyploid series has been identified in S. mahagoni in plantations in Fiji (Styles and Khosla, 1976). The evolution and geographical distribution of these genetic races is imperfectly understood. Variation in growth and form The extent of genetic variation in a wild population may be assessed by carrying out a genetic test of the chosen material. This involves the growth of material of different origins in trials either within or outwith the natural range, to compare growth attributes. Traditionally, this has primarily involved provenance and progeny tests (Zobel and Talbert, 1984). In the former, seeds collected from a number of different trees in a particular geographical region are bulked together, whereas in a progeny test, seedlings collected from individual mother trees are kept separate. Combined tests may incorporate progenies from a number of different provenances. Progeny tests allow the heritability of different attributes to be estimated which gives a quantitative indication of the degree genetic variation for the attribute concerned.

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Newton, Leakey and MesOn

Very few provenance or progeny tests have been carried out with Swietenia species, although such tests are the basis of many of the genetic improvement programmes that have now been initiated with other tropical timber species (see papers in Gibson et al., 1989). The lack of such tests partly reflects the difficulties in growing mahoganies in the presence of shoot boring moths (Hypsipyla spp). Although the growth of Swietenia spp. in Puerto Rico has been investigated in detail (Weaver, 1987; Weaver and Bauer, 1986), and provenance tests have been established (Geary et al., 1973; see also Boone and Chudnoff, 1970), there are apparently no published data describing provenance variation (but see Glogiewicz, 1986). Similarly, the National Resources Council (1991) reported that there are no active tree improvement activities with Swietenia species. The broad ecological and geographical ranges of Swietenia species, coupled with their ability to hybridize, would suggest that a high degree of genetic diversity may exist in mahoganies. In contrast to the situation with Swietenia, the genetic variation in Cedrela odorata L. (Spanish Cedar) has been investigated in some detail. The results of trials of C. odorata, which is closely related to Swietenia spp., illustrate the kind of variation which might exist in Swietenia. The main studies undertaken to date with C. odorata are the international provenance trials coordinated by the Oxford Forestry Institute, UK. The results of these trials were summarized by Chaplin (1980) (see also Burley and Lamb, 1971). In 1967, seedlots of 14 provenances were distributed to 21 collaborating countries throughout the tropics, for use in trials. In the neotropics few trials were successfully established, and problems with shoot borer attack and site incompatibility were encountered (Whitmore. 1978). The same seedlots were also tested in a number of localities in Africa, where pronounced differences in growth and form were observed between provenances, partly because of the low incidence of shoot borer attack. Provenance differences in mean height growth by up to a factor of six were recorded in these trials (see papers in Burley and Nikles, 1973; Nikles et al., 1978). In general, the most promising provenances in terms of growth were those from Costa Rica and Belize (Chaplin, 1980). As noted above, few published data of a similar nature exist for Swietenia spp. at present. However, a number of initial small-scale trials are currently in progress. As part of a collaborative link between the Institute of Terrestrial Ecology (ITE), the Centro Agronomico Tropical de Investigacion y Ensefianza (CATIE) and the International Institute of Biological Control (IIBC), a number of provenance tests and combined progeny/provenance tests have recently been established in both Costa Rica and Trinidad (Newton, 1990; Newton et al., 1991, 1992). Although some origins of S. humilis and S. mahagoni are being tested on a small scale, the principal species in this programme are C. odorata and S. macrophylla. In addition, a tree improvement group based at ESNACIFOR (Siguatepeque) in Honduras is currently establishing provenance/progeny tests of S. hurnilis. No results of these trials have been published to date. Variation in pest resistance As noted above, mahogany species are susceptible to attack by shoot borers (Hypsipyla spp.), perhaps the best known insect pest of any tropical forest tree ~see Grijpma, 1973; Whitmore, 1976a,b). The larvae of the moth bore into the apical part of the stem, destroying the terminal shoot and bud, and resulting in forking or deformation of the trunk. Attacks by shoot borers virtually prevent establishment of Swietenia in plantations in many parts of the neotropics. Although considerable research effort has been devoted


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to the development of control methods for this insect pest (Grijpma, 1974; Whitmore, 1976a,b), few practical control measures have been developed. If mahoganies are to be established in plantations to offset losses through degradation of natural stands, it is essential that the shoot borer problem is overcome. This might be achieved by a combination of silvicultural, biological and perhaps chemical control methods (Newton et ai., 1992). However, it has repeatedly been suggested that the most effective way to combat this pest would be to produce resistant plants, through selection in a breeding programme (Grijpma, 1976; Newton, 1990; Newton et al., 1992). Resistance may arise through three main mechanisms (Grijpma, 1976): (i)

non-preference, when the insect is not attracted to or is actively repelled from ovipositing or feeding on the tree; (ii) antibiosis, in which the insect is killed, injured or prevented from completing its lifecycle after feeding on the tree; (iii) tolerance, in which the tree recovers from attack to an acceptable level. Evidence for each of these mechanisms of resistance has been documented for different species of Meliaceae, but very little information is available concerning the intraspecific variation in these attributes. The fact that some mahogany species are less susceptible to attack than others is well established. In Puerto Rico, S. mahagoni is less attacked than S. macrophylla, and the hybrid S. mahagoni × S. rnacrophylla is intermediate (Whitmore and Honojosa, 1977). Similarly, Cedrela species are generally thought to be attacked more readily than Swietenia spp. (Dourojeanni, 1963; Grijpma, 1970). These differences in susceptibility may reflect variation in the production of chemical attractants or toxins, although differences in growth rate and form may also be influential (Grijpma, 1976). As noted by Grijpma (1976), there is a strong possiblity that non-preferred mahoganies may exist, which could be exploited in a selection programme. However, few studies have been made of intraspecific variation in susceptibility to attack (see Whitmore, 1978). Examples of antibiosis are provided by species exotic to the Americas, such as Toona ciliata M. Roem. This species produces water-soluble compounds which are toxic to Hypsipyla grandella (Zeller), by retarding growth and interfering with pupal development (Grijpma and Roberts, 1975). These chemical compounds are translocated to C. odorata when the latter is grafted onto Toona rootstocks, conferring resistance (Grijpma and Roberts, 1975). Furthermore, some mahoganies produce resins, which may hinder shoot borer attack (Lamb, 1968; Whitmore, 1978; Wilkins, 1972), but again, no information is available on the intraspecific variation in the production of these compounds. The ability of certain Cedrela trees to recover following attack by strong apical growth has been repeatedly observed (Chaplin, 1980; Grijpma, 1976; Vega, 1976). This kind of tolerance is at least partially under genetic control (Chaplin, 1980) and therefore could also form the basis of a selection programme (Grijpma, 1976). Although not widely recorded for Swietenia species, S. macrophylla clearly can display this pattern of response (Newton, personal observation). Pronounced genetic variation in apical dominance, the physiological process determining the relative strength of apical and lateral meristems, has been recorded for other tropical hardwood species by Leakey and Longman (1986) and Leakey and Ladipo (1987). A simple nursery test to predict the genotypic variation in apical dominance (and branchiness) of individual trees has also been developed (Ladipo et


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al., 1991a,b). This is currently being applied to Cedrela odorata (Newton, Mes6n and Leakey, 1991). Preliminary results from combined provenance/progeny tests of C. odorata in Costa Rica, undertaken as part of the I T E / C A T I E link project (Newton et al., 1991, 1992, Newton, 1990), have indicated intraspecific variation in different forms of resistance. A p a r t from pronounced differences in growth rate, different families (or progeny sharing a c o m m o n parent) displayed three-fold variation in susceptibility to attack (Fig. 1). This variation was highly significant (p < 0.001, A N O V A ) . In addition, some of those individuals which were attacked were subsequently able to recover well by vigorous growth of a new dominant lateral shoot. Further investigations are required to test whether intraspecific variation in susceptibility is caused by variation in the production of chemical attractants or toxins, and the extent to which these different forms of resistance are under genetic control. If variation in resistance occurs at the level of individual genotypes, as well as at the family level, then this can be captured by vegetative propagation (see later). 70 ¢J 6 0

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The case for genetic conservation

The conservation status of mahoganies has recently been the subject of some attention, as a result of the proposal by the United States and Costa Rican governments to list the genus Swietenia on Appendix II of the Convention on International Trade in Endangered Species (CITES). Swietenia humilis was listed on CITES Appendix II in 1973, following a proposal from Mexico. Swietenia mahagoni was listed as a result of a meeting by the Convention in 1992, but the proposal to list S. macrophylla was withdrawn prior to voting. Although concern about the status of S. macrophylla is growing (Read, 1990), little detailed information exists about the extent of native populations (US CITES proposal, 1992). The species is considered as vulnerable in many parts of its natural range because of over-exploitation (National Research Council, 1991; Palmberg, 1987), and has been listed as a high priority species for genetic conservation by the Food and Agriculture Organization (FAO, 1989; see also Palmberg, 1987) and the International Board for Plant Genetic Resources (IBPGR). The current conservation status of neotropical mahoganies is discussed in more detail by Rodan et al. (1992). Selective logging acts as a source of dysgenic selection, whereby the best genotypes (in terms of growth or form) are selectively removed during the course of forestry operations. This results in a population depleted in its most favoured genotypes (genetic erosion). Swietenia mahagoni is perhaps the most striking example of genetic erosion in tropical forestry. This species is now most commonly seen as a highly branched shrub or small tree, whereas it was formerly abundant as a large tree of good form throughout its natural range (B.T. Styles in Pennington, 1981). Genetic erosion of other Swietenia and Cedrela species has already occurred in Central and South America, where trees of good form are now rarely encountered except in isolated areas (Styles and Khosla, 1976). In all these species it is likely that the poorly formed trees left by loggers are those with little resistance to Hypsipyla, since frequent attack affects tree form. As noted in the previous section, few analyses have been made of the extent of genetic variation within mahogany species, in terms of the differences between provenances, families or individuals, or the existence of particular races. The reduction in this intraspecific genetic diversity resulting from logging remains unquantified. The potential loss of genotypes resistant to pest attack is perhaps one of the strongest arguments in favour of greater protection for remaining stands of mahogany. As described above, selection for pest resistance has great potential as a method of reducing the damage to commercial plantations caused by the mahogany shoot borer. Similarly, the possible existence of provenances or individuals with enhanced growth rates or wood quality remains largely untested. The exploration and testing of Swietenia genotypes is therefore an urgent priority. If pest resistant or otherwise superior genotypes and populations are identified, their in situ and ex situ conservation and utilization should become immediate objectives, on economic as well as biological grounds. The techniques by which such valuable or endangered genotypes might be conserved ex situ are described below. The capture of genetic variation

Selected genotypes may be 'captured' (i.e. conserved ex situ) through the techniques of vegative propagation, either through rooting leafy cuttings (macropropagation) or by meristem proliferation in vitro (micropropagation). These techniques enable selected

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genotypes to be multiplied asexually, or cloned. The domestication of mahoganies by selection for pest resistance, tree form and growth rates, and by breeding between resistant parents in clonal seed orchards, could enable progress to be made in the creation of a commercial resource of profitable mahogany plantations to take the pressures off threatened natural stands. Cloning techniques are also useful for the characterization and quantification of genetic variation, and for ex situ conservation of endangered genotypes, Looking further to the future, micropropagation techniques may contribute to genetic engineering for pest resistance and other attributes. Macropropagation

To date, more work has been done to develop techniques of rooting stem cuttings in African Mahogany (Khaya ivorensis) and African Walnut (Lovoa trichilioides) (Tchoundjeu, 1989; Z. Tchoundjeu and R.R.B. Leakey, in preparation) than in the neotropical members of the Meliaceae, although a number of different species of both the wet and dry tropics have been successfully propagated vegetatively (Table 1). From these reports from West Africa and elsewhere, it is clear that existing techniques are applicable to Swietenia spp. and to Cedrela odorata (see Leakey et al., 1982; Newton et al., 1991; Mes6n et aI., 1992; Howard et al., 1988). Much is now known about the physiology of rooting and how to manage juvenile stockplants to maximize and sustain good rooting ability, in the relatively fast growing, light demanding tropical hardwoods such as Triplochiton scleroxylon (Leakey, 1983;

Table 1. Species of the Meliaceae which have been propagated vegetatively (by macropropagation) Azadirachta indica Carapa procera Cedrela odorata

Entandrophragma angolense Khaya anthotheca Khaya ivorensis Khaya senegalensis Lovoa trichilioides Melia azedarach Melia volkensii Swietenia humilis Swietenia macrophylla Swietenia mahagoni Toona ciliata

Leakey et al. (1982) Dick et al. (1992) Addei (1977)a Britwum (1970)~ Howland and Bowen (1977) Leakey et al. (1982) Dick et al. (1992) Dick et al. (1992) Tchoundjeu (1989) Asanga (1989) Dick et al. (1992) Tchoundjeu (1989) K. Milimo (unpublished) Dick et al. (1992) J.F. Mes6n (unpublished) Mes6n etal. (1992) Howard et aL (1988) Leakey et al. (1982) Leakey et al. (1982)

apropagationby leafycuttingsby air layering.Hybridmaterialof Swietenm has also been propagated successfully(Howard et al., unpublisheddata; J.F. Mes6n, 1988).


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Leakey and Mohammed, 1985; Leakey and Coutts, 1989; Leakey and Storeton-West, 1992). It seems, however, that the requirements for rooting the shade tolerant, slower growing, quality hardwoods such as African Mahogany and African Walnut, differ in a number of important ways. For example, cuttings of K. ivorensis require about five times more auxin to maximize rooting success than T. scleroxylon, whereas L. trichilioides has a requirement for leaf area that is four times greater (Tchoundjeu, 1989). More fundamentally, perhaps, the key role of current assimilates in the rooting process of light demanding species seems to be of little importance to shade tolerant species, which appear to possess greater stored reserves of carbohydrates. It is not clear to what extent the growth of the Meliaceae by recurrent flushing, rather than free growth, is a factor in this major difference between species from different stageg of the ecological succession. In recent years, considerable effort has been made to develop cheap, low-technology propagators appropriate for use in rural development programmes in the tropics (Leakey et al., 1990). These propagators have now been established in Costa Rica and Trinidad under the ITE/CATIE link and in collaboration with IIBC, for the propagation of S. macrophylla and C. odorata. Using these propagators, preliminary tests have been made to determine the best rooting media for these two species (Mes6n et al., 1992). Percentage rooting of S. macrophylla was found to be higher when media with a high proportion of sand were used; maximum rooting of over 60% was achieved with 75:25 sand:gravel (Fig. 2a). The concentration of IBA applied to the base of the cuttings was found to have only a slight effect on rooting (Fig. 2b) (p > 0.05, t-test). Micropropagation

To our knowledge only two species in the Meliaceae, Khaya ivorensis and Cedrela odorata, have been propagated using in vitro meristem proliferation techniques. Terminal and axillary buds from young plants of K. ivorensis were relatively easy to sterilize and get into aseptic culture (Mathias, 1988). Some difficulty was, however, experienced in inducing these buds to proliferate, as there was great variation between buds in their responses to the culture medium. Shoot proliferation was greatest in explants retaining a portion of the original stockplant and when cultured on media containing 5.0 mg 1-1 benzylaminopurine + 0.05 mg 1-1 napththalene acetic acid + 0.5 mg1-1 gibbereUic acid. Success was also increased when explants were collected during the dormant stage of the stockplant flushing cycle and three to nine days after shoot decapitation, especially if the plants had been grown under light conditions with low red:far-red ratio. Second generation cultures proliferated best when the plant growth regulators in the medium were at a level below that found optimal in the first generation. Maruyama et al. (1989) described micropropagation of C. odorata by shoot-tip culture, using a medium supplemented with 6-benzylamino purine (BAP). Large rooting percentages (90%) were obtained on half-strength amounts of the same initial medium, containing 0.5mg1-1 indole-3-butyric acid and 0.05mg1-1 naphthaleneacetic acid. Although apparently so far untested, it seems likely that micropropagation of Swietenia spp. should also be possible using similar techniques (Lee and Rao, 1988). Breeding and selection Although mahoganies offer great potential for breeding programmes (Styles and Khosla, 1976), few programmes have been initiated to date. In the first instance, selection of existing variation within wild populations will probably be sufficient to make progress in


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increasing the availability of commercially acceptable planting stock, which preferably would display some degree of pest resistance. This same material would then be ideal for a future breeding programme aimed at improving pest resistance, and perhaps incorporating several forms of resistance into new progenies. A breeding programme should take into account the fact that the Swietenioideae are monoecious and little is known about their incompatibility systems (Styles and Khosla, 1976), although at least under experimental conditions, self-pollination can apparently occur (Yang, 1965; see Styles and Khosla, 1976). In a breeding programme it would be necessary to include growth parameters so that, for example, pest resistant lines could be screened for growth rate/ yield and other attributes such as wood quality. Throughout such a programme it would be very important to establish a germplasm bank, incorporating as much of the genetic variation as possible (see Leakey, 1991 for a strategy for clonal approaches to tree improvement). Use should be made of the potentail to hybridize between species, as this can increase the genetic variation within the population. It is important to remember, however, that hybrids can be infertile or may not breed true, so that a complex programme of back-crossing may be necessary to maintain genetic improvements and diversity.

Conclusion It is clear from this review of genetic variation that any conservation strategy for the mahoganies should include a well coordinated tree improvement/breeding programme as well as the protection of natural stands in situ. This programme should incorporate both new cloning techniques and traditional tree breeding, with an intensive programme of genetic exploration, inventory and testing, and would require a permanent and secure location with reliable long term funding. Such a programme would enable ex situ conservation of desirable or endangered genotypes in germplasm banks, and should increase the availability of commercially acceptable planting stock with enhanced resistance to shoot borer attack. We believe that a genetic conservation strategy following these guidelines is urgently required, if mahogany is to be maintained as a commercial resource into the next century.

Acknowledgements This paper was presented to a workshop on mahoganies organized by the Tropical Forest Foundation (TFF), Washington DC, during February, 1992. We thank the TFF for financing our participation. The ITE/CATIE link project is supported by the Overseas Development Administration (ODA), UK. References Addei, B.K. (1977) The air-layering of Treminalia ivorensis A. Chev. (Emire) and Triplochiton scleroxylon K. Schum. (Wawa) and Cedrela odorata L. (Spanish Cedar), Diploma of Forestry Thesis, University of Science and Technology, Kumasi, Ghana. Asanga, A.C. (1989) Effects of leaf area on rooting success in leafy stem cuttings of the tropical hardwood, Khaya invorensis A. Chev. (African Mahogany). MSc Thesis, University of Edinburgh. Boone, R.SI and Chudnoff, M. (1970) Variations in wood density of the mahoganies of Mexico and Central America. Turrialba 20(3), 369-71.

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Britwum, A.P.K. (1970) Notes on air layering in Cedrela odorata and Terminalia ivorensis. Tech. News. Forest Prod. Res. Inst., Ghana 4(3), 11-3. Burley, J. and Lamb, A.F.A (1971) Status of the CFI international provenance trial of Cedrela odorata (including C. mexicana and C. tubiflora). Common. For. Rev., 50(3), 234-7. Burley, J. and Nikles, D.G., eds (1973) Tropical provenance and progeny resarch and international co-operation. Proceedings of IUFRO Working Party meeting at Nairobi, Kenya. Oxford: Commonwealth Forestry Institute. Chaplin, G.E. (1980) Progress with provenance exploration and seed collection of Cedrela spp. Proceedings of the 11th Commonwealth Forestry Conference Commonwealth Forestry Institute, pp. 1-17. Oxford, UK. Dick, J. McP., Munro, R.C., Mason, P.A., Wilson, J., Ingleby, K., Muthoka, P.N., Jefwa, J., Newton, A.C. and Leakey, R.R.B. (1992) Opportunities for improving genetic quality and survival of planted trees in semi-arid lands. In Faidherbia albida in the West African semi-arid tropics; state of the art and goals for the future. Niamey, Niger: Regional Workshop, ICRISAT. Dourojeanni, R.M. (1963). E1 barreno de los brotes (Hypsipyla grandella) en cedro y caoba. Agronomia 30(1), 35-43. FAO (1989) Report of Panel of Experts on Forest Gene Resources. Food and Agriculture Organization, Rome, December 1989. Geary, T.F., Barres, H. and Ybarra-Coronado, R. (1973) Seed source variation in Puerto Rico and Virgin Islands grown mahoganies. US Department of Agricultural Forestry Service Research Papers ITF-17, Rio Piedras, Puerto Rico. Gibson, G.L., Griffin, A.R. and Matheson, A.C. (1989) Breeding tropical trees; population structure and genetic improvement strategies in clonal and seedling forestry. Proceedings of the IUFRO Conference, Pattaya, Thailand. Oxford: Oxford Forestry Institute, and Arlington, Virginia: Winrock International. USA. Glogiewicz, J.S. (1986) Performance of Mexican, Central American and West Indian provenances of Swietenia grown in Puerto Rico. MSc thesis, Syracuse, New York: State University of New York. Grijpma, P. (1970) Immunity of Toona ciliata M. Roem vat australis (F.V.M.) C.D.C. and Khaya invorensis A. Chev. to attacks of Hypsipyla grandella Zeller in Turrialba, Costa Rica. Turrialba, 211(1), 85-93. Grijpma, P. (ed.) (1973) Proceedings of the first symposium on integrated control of Hypsipyla. Turriaiba, Costa Rica: CATIE. Grijpma, P. (1974) Contributions to an integrated control programme of Hypsipyla grandella (Zeller) in Costa Rica. Wageningen, Netherlands, Landbouwhogeschool te Wageningen. Grijpma, P. (1976) Resistance of Meliaceae against the shoot borer Hypsipyla with particular reference to Toona ciliata M.J. Roem, vat australis (F.v.Muell.) C.D.C. In Tropical trees: variation, breeding and conservation (J. Burley and B.T. Styles, eds), pp. 69-78. London: Linnaean Society. Grijpma, P. and Roberts, S.C. (1975) Studies on the shootborer Hypsipyla grandella (Zeller (Lep., Pyralidae); XXVIII. Biological and chemical screening for the basis of resistance of Toona ciliata M.J. Roem. var australis (F.v.M.) C.D.C. Turrialba, 25(2), 152-9. Holdridge, L.R. and Poveda, L.J. (1975) Arboles de Costa Rica. Vol. I., San Jose, Costa Rica: Centro Ciencias Tropicales. Howard, F.W., Verkade, S.D. and DeFilippis, J.D. (1988) Propagation of West Indies, Honduran and hybrid mahoganies by cuttings, compared with seed propagation. Proc. Florida State Hort. Soc. 101,296-8. Howland, P. and Bowen, M.R. (1977) Triplochiton scleroxylon K. Schum. and other West African tropical hardwoods. Research Report, 1971-7, Nigeria: Forest Research Institute of Nigeria. Ladipo, D.O., Leakey, R.R.B. and Grace, J. (1991a) Clonal variation in a four-year-old


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plantation of Triplochiton scleroxylon K. Schum and its relation to the predictive test for branching habit. Silvae Genet. 40, 130-5. Ladipo, D.O., Leakey, R.R.B. and Grace, J. (1991b) Clonal variation in apical dominance of Triplochiton scleroxylon K. Schum. in response to decapitation. Silvae Genet. 40, 135-40. Lamb, A.F.A. (1968) Fast growing timber trees of the lowland tropics, No. 2. Cedrela odorata L. Commonwealth Forestry Institute. Lamb, F.B. (1966) Mahogany of tropical America: its ecology and management. Ann Arbor: University of Michigan Press. 220p. Leakey,R.R.B. (1983) Stockplant factors affecting root initiation in cuttings of Triplochiton scleroxylon K. Schum., an indigenous hardwood of West Africa. J. Hort. Sci. 58, 277-290. Leakey, R.R.B. (1991) Towards a strategy for clonal forestry: some guidelines based on experience with tropical trees. In Tree Breeding and Improvement (J.E. Jackson, ed.). 27-42. Tring: Royal Forestry Society of England, Wales and Northern Ireland. Leakey, R.R.B. and Coutts, M.P. (1989) The dynamics of rooting in Triplochiton scleroxylon K. Schum. cuttings - their relationship to leaf area, node position, dry weight accumulation, leaf water potential and carbohydrate composition. Tree Physiol. 5, 135-46. Leakey, R.R.B. and Mohammed, H.R.S. (1985) Effects of stem length on root initiation in sequential single-node cuttings of Triplochiton scleroxylon K. Schum. J. Hort. Sci. 60, 431-7. Leakey, R.R.B. and Storeton-West, R. (1992) The rooting ability of Triplochiton scleroxylon K. Shum cuttings: the interactions between stockplant irradiance, light quality and nutrients. Forest Ecol. Man. 49, 133-50. Leakey, R.R.B. and Longman, K.A. (1986) Physiological, environmental and genetic variation in apical dominance as determined by decapitation in Triplochiton scleroxylon. Tree Physiol. 1, 193-207. Leakey, R.R.B. and Ladipo, D.O. (1987) Selection for improvement in vegetatively propagated tropical hardwoods. In Improvement of vegetatively propagated plants. (R. Atkins and J. Abbot, eds), pp. 324-36. London: Academic Press. Leakey, R.R.B., Last, F.T. and Longman, K.A. (1982) Domestication of forest trees: a process to secure the productivity and future diversity of tropical ecosystems. Common. Forest. Rev. 61, 33-42. Leakey, R.R.B., Mes6n, J.F., Tchoundjeu, Z., Longman, K.A., Dick, J. McP., Newton, A., Matin, A., Grace, J., Munro, R.C. and Muthoko, P.N. (1990) Low-technology techniques for the vegetative propagation of tropical trees. Comm. Forest. Rev. 69(3), 247-57. Lee, S.K. and Rao, A.N. (1988) Plantlet production of Swietenia macrophyUa (King) through tissue culture. Garden Bull. (Singapore) 41, 11-8. Maruyama, E., Katsuaki, I., Saito, A. and Migita, K. (1989)Micropropagation of Cedro (Cedrela odorata L.) by shoot-tip culture. J. Jpn For. Soc. 71,329-31. Mathias, P.J. (1988) Micropropagation of the tropical hardwoods, Khaya ivorensis A. Chev. and Nauclea diderichii (de Wild and Th. Dur.) Merrill, PhD Thesis, Universily of Nottingham. Mes6n, J.F., Leakey, R.R.B. and Newton, A.C. (1992) Hacia el desarrollo de tecnicas de silvicultura clonal para el pequefio finquero. El Chasqui (CATIE) 28, 6-18. National Research Council (1991) Managing global genetic resources: forest trees. Board of Agriculture, National Research Council. Washington: National Academy Press. Newton, A.C. (1990) Selection por resistencia al perforador de las Meliaceas. Notciero (5), Mejoramiento Genetico y Semillas Forestales para America Central. Turrialba, Costa Rica: CATIE. Newton, A.C., Mes6n, J.F. and Leakey, R.R.B. (1991) 1TE/CATIE link: Interim Report to the Overseas Development Administration. Edinburgh, UK: Institute of Terrestrial Ecology. Newton, A.C., Mes6n, J.F. and Leakey, R.R.B (1992) The ITE/CATIE link. Second interim report to the Overseas Development Administration, UK. Edinburgh, UK: Institute of Terrestrial Ecology.

126

Newton, Leakey and Mes(n

Newton, A.C., Baker, P., Ramnarine, S., Mesrn, J.F. and Leakey, R.R.B. (1993) Mahogany shoot borers: prospects for control. Forest Ecol. Man. (in press). Nikles, D.G., Burley, J. and Barnes, R.D., eds (1978) Progress and problems of genetic improvement of tropical forest trees: proceedings of a IUFRO Working Party meeting, held in Brisbane, Australia. Oxford: Commonwealth Forestry Institute. Palmberg, C. (1987) Conservation of genetic resources of woody species. Simposio sobre Silvicultura y Mejoramiento Genetico, Centro Investigacirn y Estudios Forestales, Buenos Aires, April 6-10, 1987. Pennington, T.D. (1981) A monograph of the neotropical Meliaceae. New York: The New York Botanical Gardens. Pennington, T.D. and Styles, B.T. (1975) A generic monograph of the Meliaceae. Blumea 22, 419-540. Read, M. (1990) Mahogany -forests or furniture? Brighton: Fauna and Flora Preservation Society. Rodan, B., Verissimo, A. and Newton, A.C. (1992) Mahogany resource conservation: status and policy initiatives, Environ. Conserv. (in press). Styles, B.T. and Khosla, P.K. (1976) Cytology and reproductive biology of Meliaceae. In Tropical trees: variation, breeding and conservation (J. Burley and B.T. Styles, eds), pp. 61-7. London: Linnean Society. Tchoundjeu, Z. (1989) Vegetative propagation of the tropical hardwoods Khaya ivorensis A. Chev. and Lovoa trichilioides Harms. PhD Thesis, University of Edinburgh, UK. US CITES Proposal (1992) Proposal to list the genus Swietenia on Appendix H of the Convention on International Trade in Endangered Species (CITES). Washington DC: US Department of the Interior, Fish and Wildlife Service. Vega, C.L. (1976) Influencia de la silvicultura en el comportamiento de Cedrela in Suriname. In Studies on the shootborer Hypsipyla grandella (Zeller) Lep. Pyralidae. Vol. III. ( J i . Whitmore, ed.), pp. 26-49. IICA Miscellaneous publication No. 101. Turrialba, Costa Rica: CATIE. Weaver, P.L. (1987) Enrichment plantings in tropical America. In Management of the forests of tropical America: prospects and technologies (J.C. Figueroa, F.H. Wadsworth and S. Branham. eds), pp. 259-78. Puerto Rico: USDA Forest Service, Institute of Tropical Forestry. Weaver, P.L. and Bauer, G.P. (1986) Growth, survival and shoot borer damage in mahogany plantings in the Liquillo Forest in Puerto Rico. Turrialba, 36(4), 509-22. Whitmore, J.L. (1976a) Studies on the shootborer Hypsipyla grandella (Zeller) Lep. Pyralidae. Vol. If. IICA Publications No. 101, Turrialba, Costa Rica: CATIE. Whitmore, J.L. (1976b) Studies on the shootborer Hypsipyla grandella (Zeller) Lep. Pyralidae. Vol. III. IICA Publications No. 101. Turrialba, Costa Rica: CATIE. Whitmore, J.L. (1978) Cedrela provenance trial in Puerto Rico and St Croix: establishment phase. USDA Forest Service Research Note No. IRF 16. Puerto Rico: Institute of Tropical Forestry. Whitmore, J.L. (1983) Swietenia macrophylla and S. humilis (Caoba, mahogany). In Costa Rican Natural History (D.H. Janzen, ed.), pp. 331-3. Chicago and London: University of Chicago Press. Whitmore, J.L. and Hinojosa, G. (1977) Mahogany (Swietenia) hybrids. Forest Service Research Paper ITF-23. Rio Piedras, Puerto Rico: USDA Forest Service, Institute of Tropical Forestry. Wilkins, R.M. (1972) Suppression of the shoot borer Hypsipyla grandella (Zeller) (Lepidoptera, Phyticidae) with controlled release insecticides. PhD thesis, University of Washington, USA. Yang, B.Y. (1965) Study on techniques and possibilities for mahogany breeding. Bull. Taiwan For. Res. Inst. 113, 1-13. Zobel, B. and Talbert, J. (1984) Applied forest tree improvement. New York: John Wiley & Sons.